1. Field of the Invention
The present invention relates to an organic electroluminescent element, and particularly, to an organic electroluminescent element improved in durability.
2. Description of the Related Arts
The rapid diversification of data processing apparatuses in recent years has invited an increasing demand for a flat-type display element that can save electricity and space compared with a CRT(cathode-ray-tube). A crystalline liquid display element and a plasma display element are typical flat-type display elements; however, another flat-type display element, a self-luminous electroluminescent element(EL element), that enables a sharp, clear display has been attracting considerable attention recently. The EL element can be made of either organic or non-organic material, and the non-organic EL element has been already put into practical use.
The non-organic EL element emits light, or luminesces, when electrons fully accelerated in an imposed electric field collide with a luminous material and excite the luminous material. However, to accelerate the electrons to a sufficient speed, a high voltage must be applied, which undesirably increases the costs of peripheral apparatuses. On the other hand, the organic EL element luminesces when charges, or namely holes and electrons injected from their respective electrodes, re-combine in the luminous material. Unlike the acceleration, the injection of the holes and electrons requires a low voltage. In addition, with organic compounds, a desired luminous color can be created by changing a molecular structure thereof. These are the major reasons why the organic EL elements have been attracting considerable attention as being a potential display element.
For further understanding, the organic EL element will be described more in detail. The typical organic EL element has an SH-A structure, an SH-B structure, or a DH structure. In the SH-A structure, a hole-transport layer and a luminous layer are formed, one on top of the other, between a hole-injecting electrode and an electron-injecting electrode; in the SH-B structure, the luminous layer and an electron-transport layer are formed, one on top of the other, between the hole- and electron-injecting electrodes; in the DH structure, three layers--the hole-transport, luminous, and electron-transport layers--are alternatively layered between the hole- and electron-injecting electrodes. The hole-injecting electrode is made of a material having a large work function such as gold or ITO(an oxide of In and Sn), while the electron-injecting electrode is made of a material having a small work function such as Mg. All of the hole-transport, luminous, and electron-transport layers are made of organic materials; the hole-transport layer is made of the organic material of p-type, while the electron-transport layer of n-type; the luminous layer is made of the organic material of n-type in the SH-A structure, p-type in the SH-B structure, and neutral in the DH structure. In any case, the organic EL element luminesces when the holes and electrons injected from their respective electrodes re-combine in the luminous layer and at the surface contacting with the hole- and/or electron-transport layers.
The demand for practical applications of the organic EL element has been increasing. Yet, this potential display element is now facing several problems to be solved urgently to be commercially useful : one of them is improvement in durability.